1. Field of the Invention
This invention is concerned with glass transmission lines for electromagnetic energy having wavelengths in the visible or near visible spectra.
2. Description of the Prior Art
The potentialities of communications systems having large bandwidth, important in regions with high traffic density where increased communications capacity is desired, and small size, important in congested areas where space is at a premimum, have led to exploration of the feasibility of optical communications systems. While many transmission media have been proposed, those using glass transmission lines for electromagnetic energy in the visible or near visible spectra, i.e., in the region between 0.5.mu.m and 2.0.mu.m, appear most promising. In addition to extensive work done on techniques for injecting and extracting energy from the transmission lines, much work has been done on the composition and fabrication of glass fibers suitable for transmission lines. Most guiding fibers desirably have low losses from scattering and absorption and low dispersion. These losses are undesirable because they reduce the amount of transmitted energy and thereby, for a given level of injected power and receiver sensitivity, reduce the maximum fiber length possible, or for a given fiber length, require either more injected power or more sensitive receivers. Dispersion is undesirable for long fiber lengths or high data rates because it causes time broadening of light pulses as they travel along the fiber which, if sufficiently large, causes pulses to overlap and thereby precludes useful communication. Guiding is obtained with a transmission line having a core surrounded by a clad of slightly lower refractive index. The most promising compositions for the core and clad are presently silica and modified silica compositions. The core and clad may have radially uniform or varying refractive indices.
The major absorption loss is from impurity absorption, usually transition metal ion or OH radical, and is controlled by reducing impurity concentrations. The transition metal ion concentration is desirably less than 10.sup.-5 percent by weight. The OH absorption is centered around a wavelength of about 0.95.mu.m and is due to the third harmonic of an absorption band near 0.273 .mu.m. For operation near 1.0.mu.m, OH content is desirably less than 0.2 percent by weight. The scattering loss, at low power levels, is primarily from thermal fluctuations of the constituent atoms of the glass that are frozen when the glass cools below the glass transition temperature and cause density, and therefore refractive index, variations within the glass. This scattering, usually referred to as Rayleigh scattering is an intrinsic property of the material used and cannot be eliminated. It does, however, have a 1/.lambda..sup.4 dependence and losses from Rayleigh scattering decrease rapidly as the wavelength increases. An additional scattering loss often results when an oxide is added to glass to increase the refractive index of the core, as is often done, and there are concentration fluctuations of the oxide that cause refractive index variations and scattering losses.
Pulse spreading is primarily caused by material and modal dispersion. The former occurs when the refractive index of the glass varies with the wavelength of the light transmitted. The different frequency components of the light then have different velocities within the fiber and take different amounts of time to traverse a length of fiber. The latter occurs when different modes require different amounts of time to traverse a length of fiber because they have different paths along the core and accordingly, generally require different amounts of time to traverse a given length of fiber. Attempts to reduce modal dispersion have concentrated on obtaining a radially decreasing refractive index that causes the average modal velocity to increase as the modal path length increases. The increased velocity compensates for the increased path length and all modes, therefore, traverse a given length of fiber in the same time. Although these attempts have significantly reduced modal dispersion, they have not been completely successful because of theoretical and experimental difficulties encountered in producing required variation in refractive index.
Since even with the optimum radial variation in refractive index, modal dispersion is important, and because even slight deviations from the optimum cause modal dispersion to increase dramatically, there has been renewed interest in the use of single mode optical fibers which, by definition, lack modal dispersion. It may be shown, see, e.g., Proceedings of the IEEE, December 1973, pp. 1703-1751, that a fiber will propagate only a single mode when V=Ka.sqroot.2n.DELTA.n &lt; 2.405 where K=2.pi./.lambda., a is the core radius, n is the refractive index of the core and .DELTA.n is the difference between the refractive indices of the core and clad. Thus, as the core radius increases, .DELTA.n must decrease and vice versa. For practical fibers, the core radius must lie within a fairly well defined region. Very real difficulties encountered in splicing fibers with small cores make large cores desirable. However, large cores require small values of .DELTA.n which tend to make the fibers lossy. Even for the smallest practical radii for single mode fibers, the difference in the refractive indices of the core and clad is small. As an example, if .lambda. = 1.mu.m, a=5.mu.m and n=1.5; .DELTA.n is less than 2 .times. 10.sup.-3. For multimode fibers, .DELTA.n is typically between (8 and 20) .times. 10.sup.-3.
Prior single mode fibers have produced the required difference in refractive index between core and clad with either a silica core and a borosilicate clad, or a doped silica or doped borosilicate core and a silica or borosilicate clad. These compositions have drawbacks when fabrication techniques are considered. The first composition requires, because of the need for a very small change in refractive index between the pure silica core and borosilicate clad, a very low B.sub.2 O.sub.3 concentration in the clad. The low B.sub.2 O.sub.3 concentration is difficult to achieve because of difficulties in accurately maintaining the flow rate of the boron containing constituent when the chemical vapor deposition (CVD) or modified chemical vapor deposition (MCVD) techniques are used. Adding an index increasing dopant to a silica or borosilicate core, as is done for the second compositions, is not completely satisfactory because accurately controlling the small amount of dopant necessary for the difference in refractive indices is difficult. In addition, the dopant will lead to concentration fluctuations that may cause scattering losses. With some dopants, loss of dopant from the core because of its relative volatility constitutes a further problem. Much of the electromagnetic energy in single mode fibers travels in the clad rather than the core because of the small change in refractive index and the clad must have (1) sufficient thickness that the electromagnetic field has essentially zero magnitude at its outer radius to eliminate radiation or absorption by a covering material and (2) very low losses. With prior art compositions, it was difficult to deposit sufficient silica for adequate clad thickness because the substrate tube in which the deposition occurred collapsed at the high deposition temperatures needed. The problems associated with silica clads can not be eliminated, at the present time, by using commercially available silica tubing because the impurities in such tubing cause prohibitively high loss clads.